1. Field of the Invention
The present invention relates to an electric injection device used in an injection molding machine, die-cast molding machine, etc., a molding machine, and a control method for the injection device.
2. Description of the Related Art
Molding machines using molds (stationary and movable molds), such as injection molding machines, die-cast molding machines, etc., are configured to efficiently manufacture molded articles by repeating a series of molding cycles, including mold closing, injection, dwelling, mold opening, etc., in a predetermined order, based on a sequential program stored in a control unit. A molding machine of this type comprises an injection device for injecting a material into a mold and a mold clamping device configured to open and close the mold.
The injection device of an injection molding machine comprises a barrel configured to, for example, heat and melt a material, a screw inserted into the barrel, a screw rotation mechanism configured to rotate the screw, an injection drive mechanism configured to move the screw in the axial direction of the barrel, etc. The barrel is supported by a stationary-side frame, and the screw is rotatably supported by a movable-side frame. This movable-side frame is configured to be movable relative to the stationary-side frame in the axial direction of the screw by the injection drive mechanism. The electric injection drive mechanism comprises a ball screw, a servomotor for rotating the ball screw, a force transmission unit configured to transmit a thrust obtained by the rotation of the ball screw to the movable-side frame, etc.
On the other hand, the injection device of a die-cast molding machine comprises, for example, an injection unit comprising an injection sleeve supplied with a molten material by a pouring device, an injection plunger (extrusion member) inserted into the injection sleeve, a drive mechanism configured to advance and retreat the injection plunger, etc.
A twin-ball-screw injection device with a pair of ball screws comprises a pair of servomotors for rotating the ball screws individually, a pair of load cells configured to detect forces applied to respective force transmission units of the ball screws, etc. The twin-ball-screw injection device is advantageous in that it can obtain a high injection pressure and that loads on the individual servomotors, ball screws are low, and so on. However, the twin-ball-screw injection device has the problem of higher costs than those of a single-ball-screw injection device that uses only a single ball screw and single load cell.
To reduce the costs of the twin-ball-screw injection device, therefore, a proposal is made to use a load cell for the force transmission unit of one ball screw and a dummy load cell for the force transmission unit of the other ball screw, as disclosed in the following Patent Documents 1 to 3.
In an injection molding machine of Patent Document 1, for example, an injection pressure applied to each ball screw is calculated based on the ratio between the distance from one ball screw shaft on which the load cell is disposed to the central axis of the screw and the distance from the other ball screw shaft on which the dummy load cell is disposed to the central axis of the screw. According to an injection molding machine of Patent Document 2, an injection pressure is calculated by doubling an output from a pressure sensor (load cell) disposed on the one ball screw. According to an injection molding machine of Patent Document 3, a back pressure is detected by arithmetically processing the output of a pressure detecting sensor (load cell) disposed on the one ball screw, thereby achieving cost reduction.